Voltage controlled oscillators (VCOs) are used in phase lock loops (PLLs) to generate clocks having particular frequencies. PLLs are generally considered clock multipliers. For example, an input reference clock having a frequency of 10 Mhz can be multiplied by the PLL to yield an output clock signal having a frequency of 200 Mhz. Ideally, this clock multiplication would result in an output clock that is in perfect phase/frequency with the reference clock. In clock recovery systems, the PLL is used to align a particular data pattern with the output clock. In these applications, a phase frequency detector (PFD) is used to generate the proper frequency, while a phase detector (PD) is used to align the data pattern to the output clock.
FIG. 1 illustrates a conventional phase lock loop circuit 10. The circuit 10 has a phase detector (PD) 12, a phase frequency detector (PFD) 14, a loop filter 16, a voltage control filter 18, a voltage controlled oscillator 20 and a divider 22. The VCO 20 presents a signal to the divider 22. The divider 22 presents a feedback signal to the PFD 14 and the PD 12. The PFD 14 also receives a reference clock signal. The PD 12 also receives a data signal. The difference in frequency between the reference clock and the feedback signal is used to generate two control signals that are presented to the loop filter 16. The loop filter 16 presents a signal to the voltage controlled oscillator 20 in response to the control signals. During normal operating conditions, the reference clock is generally synchronized with the feedback signal. Such a synchronization is shown by the block 24.
A common type of VCO that may be used in a PLL is a ring VCO. FIG. 2 illustrates the construction of a ring VCO 30. The ring VCO 30 comprises several inverting stages 32a-32n. The inverting stages 32a-32n are connected in series. An output 34 of the last stage 32n is looped back to an input 36 of the first stage 32a with enough propagation delay to allow sufficient phase margin for an inversion. The output of each stage is shifted in phase from the previous stage. The magnitude of the shift is determined by the stage delay.
A ring VCO having outputs of individual stages accessible is generally considered a multi-phase VCO. The multi-phase VCO is attractive because it allows the use of "slower" parallel architecture. A serial high speed architecture is less desirable due to the high current and timing limitations that are required. Applications of multi-phase VCOs are illustrated in FIGS. 3A-C.
The matching of the phase shifts of each stage is critical for many phase sensitive applications. In an application where an XOR gate is used to multiply a clock frequency (e.g., FIG. 3B), if the phases are not matched well, a 50% duty cycle output may not be possible. In a phase detector (PD) application (e.g., FIGS. 3A and 3C) , a phase mismatch will result in static phase error, and/or jitter.
In order to reduce phase mismatch, the conventional approaches compensate for differences in stage delay. The compensation has included additional components or variation in the placement of the stages. FIG. 4 is a diagram illustrating a conventional method where compensation for phase mismatch is accomplished by matched resistors and matched capacitive loading by the addition of line capacitance to compensate for load mismatch. This method only compensates for a linear silicon gradient. Interconnect lines are not matched and critical matching elements are not localized.
FIG. 5 is a diagram illustrating placement of stages to equalize interconnect line capacitance. This method matches the interconnect lines. No compensation for silicon gradient is provided. Additionally, critical matching components are not localized.
FIG. 6 is a diagram illustrating variation in the placement of the stages to compensate for the layout gradient. Variation in stage placement only compensates for a linear silicon gradient. The interconnect lines are not matched. The critical matching elements are not localized.
FIG. 7 is a diagram illustrating centroiding the stages to reduce phase mismatch. This method requires separate power buses and power supply voltage drop matching across rows. The critical nodes (control nodes) are spread across switching nodes. The critical matching elements are not localized.
FIG. 8 is a diagram illustrating the use of a 2.times.VCO and a divider to get a 50% duty cycle clock signal without correcting for phase mismatch. A VCO running at 2.times. requires high power.
The conventional methods compensate only for a linear silicon gradient or match interconnect lines. The critical matching elements remain distributed among the stages. The distributed loads must be larger to phase match the stages.